PSU Questions

[Sub-arc]

I have a load of spare power supply units kicking about and I need one for a bass fx pedal. Problem is they give out more DC volts than they say. ie 1 is mains in and says 10.2 dcv out but it actually gives out 14.8 dcv.
Another says 9 dcv but gives out 13.7 dcv.

I was just going to ask if there was an easy way to reduce the output by a few volts as the 2 fx pedals I have (like most) are 9v DC regulated.

I think this may not be an easy fix. So the other question is, could I run the 2 pedals off the one psu by splicing a second connector off the output of the psu ?
The 9v psu I have, is rated 500mA. One pedal says power consumption is 25mA the other doesn't say the pc, but does say 'requires 9v >50mA. This pedal also runs on 4 AA batteries (6v) so its power consumption can't be that high.

There are daisy chain psu's available to run 6 pedals at the same time and these are rated at about 1300mA's so that's about 215mA per pedal.

What do ye think. What damage could I do if it all goes horribly wrong ?

 

[rtbcomp]

Assuming the PSUs are non-regulated the output figures will be no-load, the voltage will drop when load is applied, however if they are capable of supplying more current than the devices require the voltage may not drop enough to operate the pedals safely.

To do the job properly you need to regulate the outputs, it may be cheaper to buy a regulated supply rather than faff about with extra circuits.

Personally I use re-chargeable batteries, less cable and no hum!

 

[madkayaker]

The unloaded value will be higher but once there is load should drop

 

[Sub-arc]

One of the spare psu's I have I opened up and apart from the transformer, there is a small pcb with a relatively big capacitor, some resistors and something I don't recognise(5x20mm glass tube with a circuit in it) like a old valve, so I reckon this is regulated.

Now, if I open the pedal and measure the volts, post the female dc jack plug, I should see a voltage drop when its on,yes/no ?

 

[rtbcomp]

There should be a rectifier in there too. Also check the polarity and that PSU give a DC output.

 

[Sub-arc]

There should be a rectifier in there too. Also check the polarity and that PSU give a DC output.

Its deffo DC output. Its from an old pair of puter speakers which could also run on batts.

 

[Luke]

If you're not shy of a bit of soldering, a 7809 regulator will sort you out.

It looks like your current requirements are low, but if you need more current, you can parallel up regulators.

They're also available in 5v and 12v outputs. They all need a supply voltage greater than the output votlage, but a couple of volts or so.

 

[shenion]

One of the spare psu's I have I opened up and apart from the transformer, there is a small pcb with a relatively big capacitor, some resistors and something I don't recognise(5x20mm glass tube with a circuit in it) like a old valve, so I reckon this is regulated.

Now, if I open the pedal and measure the volts, post the female dc jack plug, I should see a voltage drop when its on,yes/no ?

That 5x20 glass tube will be a fuse. The supply does not look regulated as there will usually be several parts and a heatsink.

You probably won't see much of a voltage drop with the pedal on as they don't draw much.

You can run two pedals off one supply if they are the same. Different pedals may be grounded differently and cause issues if plugged into the same amp (or 2 grounded amps.)

 

[Sub-arc]

I really don't think the glass tube is a fuse. It's like a tiny old valve with one end rounded and the other end sealed with a couple of wires out of that end. Then there is some sort of ic in the middle of the tube. My first thoughts were diode.

I tried the measurements from inside the pedal and they were reduced by the following:-
psu giving out 13.5V down to 11.2V
psu giving out 9V down to 7.9V

Scott, I think you're right about been able to run the 2 off the one ps and I'll try that next. I'd prefer to fry the psu that the pedals though.
Re the daisy chain one rated 1.3A It says you can run 6 of any type of 9V pedal, so 2 off a .5A rated ps shouldn't be a problem.(he says hopefully)

 

[voipio]

Could you supply any images of the glass tube?

 

[bump]

So long as the current is high enough, don't worry about the voltage being a bit on the high side. Boss pedals are rugged things designed to work under pretty much any condition. Over the years I've had them hooked up to various supplies according to what was to hand in x,y,z venue in a country I wasn't native in.

Currently I've got 5 pedals running off one supply that also feeds the TU-1000.

 

[Wedg1e]

I really don't think the glass tube is a fuse. It's like a tiny old valve with one end rounded and the other end sealed with a couple of wires out of that end.

Sounds like a neon lamp.

 

[Sub-arc]

Here's a pic of the aforementioned tube thingy. Best pic of a bad lot.
On closer inspection, I think its a thermostat.
Thanks for the inputs folks.

 

[voipio]

It might well be a thermal fuse with the wires coming into it joined by a low melting point alloy, possibly bismuth-based. Bismuth itself forms quite distinct crystals which might look like semiconductor chips at first glance. The crystal structure may be preserved to some extent in it's alloys.

 

[Sub-arc]

I used the wrong word ^. I reckon it's a thermal cut-out not a thermostat.

A largish, spring steel type plate, with one contact point connected to 1 wire and a smaller contact point to the other wire. I've seen thermal cut-outs before, just never encased in glass.